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Transcription and Translation Supplemental Instruction Iowa State University Leader: Course: Instructor: Date: Brandon Biol 212 Dr. Coffman/Manz 3/9/2017 1. How does transcription differ in eukaryotes in comparison to bacteria? - Eukaryotes have three polymerases – RNA polymerase I, RNA polymerase II, RNA polymerase III o Each polymerase transcribes only certain types of RNA in eukaryotes. o RNA polymerase II is the only polymerase that transcribes protein-coding genes. - Promoters in eukaryotic DNA are more diverse than bacterial promoters. o Eukaryotic promoters include the TATA box and other important diverse sequences. - Eukaryotic RNA polymerases recognize promoters using basal transcription factors instead of a sigma protein. - Termination of eukaryotic protein-coding genes involve a poly-A signal. 2. Describe the process of RNA splicing. - SnRNPs (small nuclear ribonucleoproteins) bind to the start of the intron marked by bases G and U, and bind towards the end of the intron marked by base A. - Other snRNPs assemble with the initial SnRNPs bound to bases G and U, and bound to base A. This assembly forms a spliceosome. - The intron is cut between the G base and the U base, forming a loop and a single stranded stem of the intron called a lariat. - The lariat is released, the exons that were located on each side of the intron join together via a phosphodiester linkage. o This produces the continuous coding sequence known as mRNA. 3. What is the function of the 5’ cap and the poly(A) tail in mRNA? - Protect mRNAs from degradation by ribonucleases and enhance the efficiency of translation. o Ribonucleases = enzymes that degrade RNA. 4. Compare and contrast where and when the process of translation occurs in bacteria and eukaryotes. - In bacteria, translation occurs at the same time that transcription occurs o This happens because there is no nuclear envelope to separate the processes. - In eukaryotes, translation and transcription occur at separate times and in separate locations. 1060 Hixson-Lied Student Success Center 515-294-6624 [email protected] http://www.si.iastate.edu o Transcription is conducted in the nucleus to produce a mature mRNA; the mRNA is then exported to the cytoplasm of the cell where translation occurs. 5. What is the function of tRNAs? - Transfer amino acids from the RNA to a growing polypeptide. - tRNAs act as an interpreter during translation o This allows amino acids to interact with an mRNA template. 6. The overall structure of tRNA is divided into three substructures. a. Describe the primary structure of tRNA. o tRNA sequences are relatively short, ranging from 75 to 85 nucleotides in length. b. Describe the secondary structure of tRNA. o tRNA has stem and loop structures via the formation of hydrogen bonds between complementary bases. The stems are short stretches of double-stranded RNA. The loops are single stranded. o *A CCA sequence at the 3’ end of each tRNA offers a site for amino acid attachment, while a triplet on the loop at the other end of the structure could serve as an anticodon. *Anticodon = set of three ribonucleotides that forms base pairs with the mRNA codon. c. Describe the tertiary structure of tRNA. o tRNAs fold into an upside-down L shape. 7. What allows the correct amino acid for a particular tRNA to be attached? - Aminoacyl-tRNA synthetases o Catalyze the addition of amino acids to tRNAs. - ATP o Required to attach an amino acid to a tRNA. - *Each aminoacyl-tRNA synthetase has a binding site for a particular amino acid and a particular tRNA; so small differences in the shape and the base sequence of tRNAs allow aminoacyl-tRNA synthetases (enzymes) to recognize the correct tRNA for the correct amino acid. 8. Fill in the blanks to make the following paragraph correct. Ribosomes can be separated into ___two___ major substructures called the ___large___ subunit and the ___small____ subunit. The ___small____ subunit holds ___mRNA___ in place during ___translation___. The ___large_____ subunit is where ____peptide-bond formation_____ takes place. 9. Describe the three step protein synthesis sequence by ribosomes? 1. An aminoacyl tRNA diffuses into the A site; if its anticodon matches a codon in mRNA, it stays in the ribosome. 2. A peptide bond forms between the amino acid held by the aminoacyl tRNA in the A site and the growing polypeptide, which was held by a tRNA in the P site. 3. The ribosome moves down the mRNA by one codon, and all three tRNAs move one position within the ribosome. The tRNA in the E site exits; the tRNA in the P site moves to the E site; and the tRNA in the A site switches to the P site.